The present invention relates generally to blind fasteners. More specifically, the present invention relates to a blind fastener having an improved arrangement for a shear ring providing a locking collar.
Typical blind fasteners, which are commonly used to fasten components in which only one side of the workpiece is accessible, generally comprise four essential elements: a mandrel, a tubular sleeve surrounding the mandrel, a shear ring for upsetting the sleeve to form a blind head and a locking collar. In operation, the mandrel and its surrounding tubular sleeve is inserted through a hole in the components to be fastened together. An installation tool is used to pull the mandrel axially away from the workpieces, while simultaneously bearing down on the tubular sleeve. During such pulling, the shear ring axially compresses the sleeve, causing it to bulb and form a blind head. Continued pull on the mandrel forces the locking ring into a recess in the sleeve so as to lock the mandrel and the sleeve together. As a final step, the serrated end of the mandrel breaks away to complete installation of the fastener.
Summerlin, in U.S. Pat. No. 3,390,601, describes a blind fastening device comprising a tubular rivet body, a mandrel having an upsetting head adapted to be drawn into the tail end of the rivet to expand the rivet body and form a blind head, and a separate shearable member. The shearable member comprises a main portion which has an easy clearance fit over the upsetting head, and an inwardly tapered portion which converges over, and is closely fit around the mandrel stem to facilitate entry into the rivet body. As the mandrel stem and the shear ring are drawn into the fastener body, the shear ring enlarges the fastener body tail when force is applied to the stem to form the blind head. Then, as the stem is further drawn into the fastener, the tapered portion of the shear ring shears and continues to advance within the fastener body until it reaches an internal shoulder therein. As a final step, a locking collar, threaded over the mandrel stem, is forced into the head of the fastener and the stem is broken away.
Matuschek, in U.S. Pat. No. 4,012,984 describes a blind rivet assembly with a locking collar assembled on the rivet stem, within the rivet body. The rivet stem extends through the rivet body and has a pulling portion which can be gripped by a conventional rivet tool. The tail end of the stem has a tail former for expanding the tail end of the rivet body into an enlarged tail so as to secure the superposed plates together. An annular locking groove is provided around the stem, at a distance from the tail former, so that when the workpieces are riveted together, the locking groove is still within the rivet body. A recess is provided near the head of the rivet body so that when the skirt of the locking collar abuts against the driving anvil the force of pull exerted on the stem causes the locking collar to bulge outwardly so as to interlock the stem and the rivet body.
Shear rings integrally formed with the mandrel stem suffer from many manufacturing disadvantages. In order to form a shear ring integral with the stem, one must first begin with a piece of material having a diameter at least equal to the desired size of the shear ring. The stem material must then be machined using cutting tools to form the shear ring, a process which is not only time consuming, but also produces a large amount of scrap, or wasted material. In addition to the difficulties associated with the actual formation of the integral shear ring, there are also problems associated with tool wear. As the cutting tools become dull and worn, slight variations in the dimensions of the shear ring tend to appear. This is undesirable in that such variations make the fastener somewhat less reliable because the forces required to bulb the sleeve and shear the shear ring are less predictable. Thus, frequent tool sharpening or tool replacement is needed.
Because the shear ring of a blind fastener is designed to shear at a predetermined force after forming the blind head, limitations are necessarily imposed on the axial width of the shear ring, and thus accuracy of machining is important. Prior integral shear rings have been manufactured with an included angle at the base of the shear ring, proximate the mandrel stem. This included angle portion of the shear ring presents a cutting surface on the leading edge of the shear ring. Forming the shear ring having an included angle proximate the stem creates an undercut portion along the leading edge and presents forming difficulties. In operation, this type of shear ring first gouges the tail end of the sleeve so as to create a wedge therein. This wedge facilitates bulbing of the sleeve. After forming a blind head, the shear ring is sheared from the stem. Such an operation requires a large initial pulling force, and thins the walls of the tail end of the sleeve.